Increasing amount of threads actually slows down the cracking process

[mrDoctorWho]

Hello,

I'm trying to find a password for my dump of the LUKS2 header from a device. The header and dictionary are located in tmpfs.

Here is how I run the cracker:

$ bruteforce-luks -t 4 -f dict.dict -v 30 sda3-dump

And here is the speed of cracking compared having the different amount of threads.

Threads	Passwords/sec
1	0.933333
2	1.400000
3	1.500000
4	1.466667
10	1.379310
24	0.800000

As you can see, when the thread amount gets higher than 3 the program starts to slow down.

I'm running the tool on Ubuntu 20.04, on a 12-core AMD Ryzen 5900X with 32 GB of RAM.

[glv2]

This is probably caused by some lock contention in the read_dictionary_line or generate_next_password functions.

The program could be improved by generating/reading the next passwords to try in a dedicated thread...